A method for growing organic semiconducting crystals onto a surface could lead to better flexible electronic devices and video displays, researchers claim.

Transistors made from thin films of organic semiconductor crystals are crucial to flat panel displays, as well as prototype flexible electronics.

The new "block printing" technique can grow individual crystals on top of a surface previously patterned with metal electrodes. This provides a cheaper and simpler way to create circuitry on a surface, the technique's creators say.

Until now, the only practical way to add electronic components to a surface has involved completely covering it with a semiconducting material onto which these features have previously been added. This is inefficient as the components are often widely dispersed across the surface. Block head

"This work demonstrates for the first time that organic single crystals can be patterned over a large area without the need to laboriously fabricate transistors one at a time," says Zhenan Bao, who developed the technique with colleagues at the University of California, and Stanford University, also in California, US.

A polymer "printing block" is first used to stamp out the desired pattern of semiconducting crystals in the form of an "ink" made from a crystal growth agent called octadecyltriethoxysilane.

A vapour of organic semiconductor is then passed over the freshly printed material at several hundred degrees Celsius. This causes crystals to grow wherever there are dots of growth agent are deposited, potentially joining up the electrodes to produce working transistors.

"This method allows us to grow large arrays of organic single crystals directly onto flexible substrates," Bao told New Scientist. She says it could improve the performance of flexible electronics as well as some conventional ones. Tight curve

The block printing method was used to make transistors that can be three times as fast as those made using existing methods. In experiments, the researchers also deposited arrays of transistors onto a flexible surface that functioned even when bent into a curve tight enough to wrap around a pen.

The approach "could show the way to high-performance electronic devices that extend over large and flexible surfaces," writes electrical engineer Paul Heremans in an accompanying commentary article published in the same edition of Nature. "Bao and colleagues have just presented us with a new baby."

Heremans however warns that some refinements are needed if reliable transistors are to be printed using the method: "The control of crystal size and orientation will require further attention."